Cancer is a major disease burden and one hope of changing this is to develop new treatments through a better understanding of the disease. Recent discoveries concerning the activities of short RNAs in mammals may provide both new insights and new treatments of cancer. A common goal of this Program is to investigate the roles of short RNAs, such as microRNAs, in regulation of genes in normal and cancer cells. There is strong and rapidly growing evidence suggesting that changes in miRNA regulation are related to malignant transformation and in fact could be a critical event in oncogenic transformation. The function of the mir-17-92-1 cluster which is frequently overexpressed/amplified in a subset of human cancers will be investigated by creation of specific mutations of these microRNAs in the context of mouse models of cancer. Changes in microRNA populations in normal cells and tumor cells of the same developmental state will be analyzed using both bead-array technology as well as new cloning technology. Vectors with regulated expression of a short hairpin RNA which generates a specific siRNA for silencing a gene will be developed for transgenic analysis of pathways. Additionally, methods will be tested for screening of small libraries of shRNA-lentiviral vectors to identify genes which, when silenced, either inhibit or stimulate tumor development. Furthermore, libraries of retro viral vectors expressing shRNAs will be used in screens to identify (a) genes that modulate the proliferation and/or survival of pRB-deficient cells , (b) genes that modulate the rate of development of a K-ras-driven lung cancer model, and (c) genes important for the differentiation of ES cells. The potential role of short RNAs in transcriptional silencing will be investigated in embryonic stem cells. These processes could be important for epigenetic silencing and genomic stability of cancer cells. ES cells will also be studied for the role of miRNAs in development and proliferation. Changes in the spectrum of microRNAs and siRNAs during T-cell development will be characterized using a cloning technology which requires small amounts of RNAi. Activation of the Arf promoter is an early signal in oncogenic transformation. This promoter is silenced under normal conditions by the E2F3B protein, linking the p19Arf-mdm2-p53 pathway to the p16INK4a-cycD/cdk4-pRB-EdF pathway. The role of E2F3B complexes and other E2F factors in regulation of the Arf promoter will be studied.